FDDB125: Computed key property has redundant Prefix

Field	Value
Code	FDDB125
Severity	Error

Message

Property '{0}' is a computed key with Prefix = "{1}" configured on its key attribute. Prefixes are not applied to computed keys — remove the Prefix and embed it in the [Computed] Format if the prefix should appear in the stored value

Description

A computed key derives its value entirely from [Computed] configuration. The Prefix on [PartitionKey] or [SortKey] is silently ignored at runtime because the source generator's key prefix application logic excludes computed keys. This creates a confusing situation where a developer declares a Prefix expecting it to appear in the stored DynamoDB value, but it never does.

Remove the Prefix to avoid confusion, or use Format = "PREFIX#{0}" on [Computed] if the prefix should appear in the stored value.

Example

The following code triggers this diagnostic:

[DynamoDbTable("events")]
public partial class Event
{
    [PartitionKey(Prefix = "EVT")]       // ❌ Prefix is ignored on computed keys
    [DynamoDbAttribute("pk")]
    [Computed("Year", "Month", Separator = "#")]
    public string Pk { get; set; } = string.Empty;

    [Extracted("Pk", 0)]
    public int Year { get; set; }

    [Extracted("Pk", 1)]
    public int Month { get; set; }
}

Fix

Remove the Prefix from the key attribute. If the prefix should appear in the stored value, embed it in the [Computed] Format instead:

[DynamoDbTable("events")]
public partial class Event
{
    [PartitionKey]                        // ✅ No prefix on computed key
    [DynamoDbAttribute("pk")]
    [Computed("Year", "Month", Format = "EVT#{0}#{1}")]  // Prefix embedded in Format
    public string Pk { get; set; } = string.Empty;

    [Extracted("Pk", 0)]
    public int Year { get; set; }

    [Extracted("Pk", 1)]
    public int Month { get; set; }
}

Or if no prefix is desired:

[DynamoDbTable("events")]
public partial class Event
{
    [PartitionKey]                        // ✅ No prefix
    [DynamoDbAttribute("pk")]
    [Computed("Year", "Month", Separator = "#")]
    public string Pk { get; set; } = string.Empty;

    [Extracted("Pk", 0)]
    public int Year { get; set; }

    [Extracted("Pk", 1)]
    public int Month { get; set; }
}